1. Statement of the Technical Field
The inventive arrangements relate generally to the field of communications, and more particularly to frequency selective surfaces and phased array antennas.
2. Description of the Related Art
Existing microwave antennas include a wide variety of configurations for various applications, such as satellite reception, remote broadcasting, or military communication. The desirable characteristics of low cost, light-weight, low profile and mass producibility are provided in general by printed circuit antennas. The simplest forms of printed circuit antennas are microstrip antennas where flat conductive elements are spaced from a single essentially continuous ground element by a dielectric sheet of uniform thickness. An example of a microstrip antenna is disclosed in U.S. Pat. No. 3,995,277 to Olyphant.
These antennas can be designed in an array and may be used for communication systems such as identification of friend/foe (IFF) systems, personal communication service (PCS) systems, satellite communication systems, and aerospace systems, which require such characteristics as low cost, light weight, low profile, and a low sidelobe.
The bandwidth and directivity capabilities of such antennas, however, can be limiting for certain applications. While the use of electromagnetically coupled microstrip patch pairs can increase bandwidth, obtaining this benefit presents significant design challenges, particularly where maintenance of a low profile and broad beam width is desirable or where a dynamically manipulated beam is desirable. Also, the use of an array of microstrip patches can improve directivity by providing a predetermined scan angle. However, utilizing an array of microstrip patches presents a dilemma. The scan angle can be increased if the array elements are spaced closer together, but closer spacing can increase undesirable coupling between antenna elements thereby degrading performance.
Furthermore, while a microstrip patch antenna is advantageous in applications requiring a conformal configuration, e.g. in aerospace systems, mounting the antenna presents challenges with respect to the manner in which it is fed such that conformality and satisfactory radiation coverage and directivity are maintained and losses to surrounding surfaces are reduced. More specifically, increasing the bandwidth of a phased array antenna with a wide scan angle is conventionally achieved by dividing the frequency range into multiple bands.
One example of such an antenna is disclosed in U.S. Pat. No. 5,485,167 to Wong et al. This antenna includes several pairs of dipole pair arrays each tuned to a different frequency band and stacked relative to each other along the transmission/reception direction. The highest frequency array is in front of the next lowest frequency array and so forth.
This approach may result in a considerable increase in the size and weight of the antenna while creating a Radio Frequency (RF) interface problem. Another approach is to use gimbals to mechanically obtain the required scan angle. Yet, here again, this approach may increase the size and weight of the antenna and result in a slower response time. The present invention utilizes a reconfigured frequency selective surface to avoid many of these detriments.
A frequency selective surface is typically an array of periodic elements used to tightly couple resonant elements such as dipoles, slots and spatial filters that reflect. A frequency selective surface is also considered a construction that either passes or reflects certain frequencies.
Thus, there is a need for a frequency selective surface as well as a lightweight phased array antenna with a wide frequency bandwidth and a wide scan angle utilizing such frequency selective surface, and that can be conformably mountable to a surface if required. Such a need has been met through the use of current sheet arrays or dipole layers using interdigital capacitors that increase coupling by lengthening the capacitor “digits” or “fingers” that result in additional bandwidth as discussed in U.S. Pat. No. 6,417,813 to Durham ('813 Patent) and assigned to the assignee herein. Some antennas of this structure exhibit a significant gain dropout at particular frequencies in the desired operational bandwidth, spurious resonances, and possibly other undesirable characteristics. Being able to change the phase response or the resonant frequency across the frequency selective surface can likely remove most of these undesirable characteristics. Thus, a need exists for a lightweight phased array antenna with a wide frequency bandwidth and wide scan angle that overcomes the gain dropout and other undesirable characteristics discussed above.
The key to broad-band performance with a phased array antenna incorporating a frequency selective surface is to achieve constant impedance over a wide frequency range. None of the constituent components of such an array (e.g. the elements, the unit cell spacing, the mutual coupling, the dielectric properties of the material layers in which the array is embedded, and the spacing between the array and the ground plane, if any) have this constant impedance property. However, the impedance properties of the components all vary differently with frequency. With appropriate choices in accordance with the invention, these individual variations can be made to balance over a broad frequency range, so that collectively, but not individually, the design elements of the array achieve broadband performance. Note that this design approach utilizes the coupling between the elements, whereas in other array designs the coupling is considered undesirable.
In practice, the present state of the art in such arrays is limited to about 10:1 bandwidth. This is much broader bandwidth than has been achieved with other arrays, but there are applications which could benefit from even more bandwidth. The limitations in practice arise from a number of factors, including undesired resonances in the array design, e.g. in the coupling structure, and the desired scanning performance of the array. Embodiments in accordance with the present invention utilize fluids to extend the range over which the array operates, allowing the instantaneous bandwidth of the array to be utilized over an even wider operating range. Examples of the array parameters which could be affected by fluids are the coupling structures, the element resonances, and the effective ground plane spacing.